Styrene and isobutylene are two monomers which are difficult to copolymerize satisfactorily, as styrene does not generally polymerize well cationically (but quite well anionically or through radical polymerization) while isobutylene essentially polymerizes only under cationic conditions. Attempts at cationic polymerization of styrene have generally led to low molecular weight materials due to the prevalence of chain transfer reactions and thus copolymerizations with isobutylene have hitherto not afforded materials with satisfactory properties.
The cationic copolymerization of styrene with isobutylene to afford copolymers containing both monomers in the same chain has been studied extensively by Okamura et al who reported in the Journal of Polymer Science, C-16, pp 2365-2377 (1967) that, although copolymers can be obtained, the prevailing cross-transfer reactions generally lead to low molecular weight materials. This process cannot be used to prepare graft copolymers.
Fodor et al, J. Macromol. Sci. Chem. A 24, 735-47 (1987) have recently reported on the preparation of block copolymers of isobutylene and styrene by sequential cationic polymerization of the monomers at -90.degree. C. However, the polymers which were obtained lacked purity and their molecular weights could not be controlled. The block copolymers which were obtained generally had low molecular weights and poor mechanical properties. This process was not applicable to the preparation of polyisobutylene grafts onto a polystyrene backbone.
Kennedy et al, Polymer Bulletin, 13, pp 343-348 (1985) have also described the preparation of a graft copolymer of styrene and isobutylene by the macromer technique. In this process a polyisobutylene macromer terminated by a styrene residue was prepared, then copolymerized with styrene or methyl methyacrylate. However, the ability of the polyisobutylene macromonomer to copolymerize with styrene was found to depend on its molecular weight as evidenced by large variations in its reactivity ratio depending on conditions. In addition, the microphase separation which occurred at higher conversions prevented the regular distribution of the macromer units throughout the polystyrene chains. In a further report, Kennedy et al [Polymer Bulletin 13, pp 441-446 (1985)] describe the same copolymerization of styrene-terminated polyisobutylene with styrene in an aqueous emulsion. Here again the final polymer appears to have segregated polystyrene and polyisobutylene phases as the copolymerization is not homogeneous. This technique, which is limited as it cannot be applied to a broad spectrum of compositions, morphologies, and molecular weights of the two chain components, nevertheless leads to copolymers having improved mechanical properties.
In Kaszas et al, Journal of Macromolecular Science Chemistry, 1987, Vol. A-18, pp. 1367-82 the forced ideal copolymerization of isobutylene with styrene under quasi-living carbocationic conditions is described. This technique affords copolymers in which both monomers are in the same chain rather than grafted. Only a few compositions can be achieved and the process lacks versatility and is difficult to carry out.
U.S. Pat. No. 4,107,238 (1978) (Roper et al) reports on an anionic grafting of styrene onto cyclopentadiene-isobutylene-containing rubbers. The complex lithiation process involving butyl lithium and tetramethylethylene diamine is not directly applicable to the grafting of standard isobutylene fragments to polystyrene. German Patent No. 2,236,384 (1973) (Marek et al) describes a process which may be used to prepare copolymers containing some styrene and isobutylene in the same chain using titanium or vanadium based catalysts and irradiation. A complex chemical modification procedure based on a phenyl terminated polyisobutylene is described in German Patent 2,161,859 (1972) (Jean Peryot) to prepare a "graft" copolymer (actually a block copolymer) containing one segment of polyisobutylene linked to one segment of polystyrene. Although this process is limited in its versatility, since true grafts of polyisobutylene onto polystyrene cannot be formed, it is nevertheless interesting as it describes a technique allowing the growth of a polystyrene chain from the extremity of a phenyl-terminated polyisobutylene.
F. Cramer et al, Angew. Chem., Int. Ed. English, Vol. 5, 1966, page 601, describe the synthesis of a polymer containing a monomethoxy-trityl pendant group by benzoylation of polystyrene followed by reaction with a Grignard reagent. The polymer was used as a carrier for oligonucleotide synthesis.
H. Hayatsu et al, J. Am. Chem. Soc. Vol. 88, 1966, pages 3182-3183, report the preparation of exactly the same polymer as F. Cramer et al by essentially the same route. This was also reported more fully by the same author in J. Am. Chem. Soc. 89, 1967, 3880-87. In addition, L.R. Melby et al, J. Am. Chem. Soc. Vol. 89, 1967, 450-3 report the preparation of the same polymer as F. Cramer from a crosslinked insoluble polystyrenedivinylbenzene resin. Synthesis is via lithiation (n-butyl lithium). The polymer is used in oligonucleotide synthesis employing the chloride derivative as support. Finally, J. M. J. Frechet and K. E. Haque, Tetrahedron Letters 1975, pages 3055-3056, report the preparation of a crosslinked styrene-divinylbenzene resin containing ##STR3## groups by benzoylation of the insoluble polystyrene resin followed by a Grignard reaction.